SYNTHESIS OF INORGANIC NANOFIBERS AND LAMELLAR STRUCTURES WITH LARGE SPECIFIC SURFACE BY MEANS OF CONTROLLED VACUUM FREEZE-DRYING PROCESS

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1 SYNTHESIS OF INORGANIC NANOFIBERS AND LAMELLAR STRUCTURES WITH LARGE SPECIFIC SURFACE BY MEANS OF CONTROLLED VACUUM FREEZE-DRYING PROCESS Richard DVORSKY a, Jana TROJKOVÁ a, Jiří LUŇÁČEK a, Kateřina PIKSOVÁ b, Ondřej ČERNOHORSKÝ b a VSB-Technical University of Ostrava, 17. listopadu 15, Ostrava-Poruba, Czech Republic b Czech Technical University in Prague, V Holešovičkách 2, Praha 8, Czech Republic Abstract Inorganic nanopowders are utilized among others in field of chemisorption and materials where a large specific surface area is required. Materials like activated carbon with extreme density of micropores and nanopores dominate here, as well as the materials with intrinsic lamellar structure. Phylosilicates as montmorilonite, vermiculite etc., further intercalated, can be mentioned as an example. Other forms of inorganic materials like fibers and lamellae acquire large values of specific surface areas only when their characteristic sizes decrease into submicron scales. The paper reports on the properties and the method of preparing such inorganic nanostructures. They are produced in the temperature and vacuum controlled regime of freeze-drying process from an aqueous dispersion of primary nanoparticles which can be modified with a surface layer of active molecules. Keywords: nanoparticle, vacuum drying, lyophilization, inorganic nanofibers, lamellar structures INTRODUCTION At present, inorganic nanopowders are utilised to adjust surface interaction with the surroundings, in the area of catalysis and chemisorption, and for materials where a large specific surface area is required [1.-3.]. The natural materials of this kind are e.g. the activated charcoal, which has the considerable density of micropores and mesopores, and the materials with intrinsic lamellar structure such as montmorillonite, vermiculite, and other phyllosilicates, often further intercalated [4.]. When the nanopowder is prepared by means of the disintegrator WJM (Water Jet Mill) [5.], the subsequent desiccation of the output aqueous dispersion of the disintegrated material is crucial. During the ordinary hightemperature drying of the wet filtrate, the intense reaggregation of the milled nanoparticles takes place. The mean distance between the particles in a wet filter cake is very short, so the probability of their collision due to Brownian motion is high. Thus the probability of the direct contact and the activation of the bonds between the interfaces rises, which results into irreversible aggregation of the nanoparticles into larger aggregates. The alternative method of vacuum freeze-drying is used whenever the aggregation must be prevented and the dry powder of separated primordial nanoparticles is the desired product. However, at submicron level the agglutination is still present due to Van der Waals forces at the interfaces. Such agglutinated nanoparticles can be so closely packed in the given volume that e.g. the diffusion of the gas which should be adsorbed at their surfaces is substantially reduced. On the basis of our experience with the temperature and vacuum controlled regime of vacuum freeze-drying process, we have developed the fixation method which converts the individual nanoparticles into solid fibrillar and lamellar structures with large specific surface area. 1. PHYSICAL BACKGROUND Mutual interactions of the dispersed nanoparticles during the vaporization of water from the liquid or solid (frozen) phases are very different and they have large influence on the character of the final dry powder. The fundamental parameters affecting the creation and the stability of the bounds between the nanoparticles are:

2 A) The magnitude and direction of the relative velocity of the particles at the instant of collision. In the case of liquid dispersion of the filter cake they are determined by the temperature-velocity dependence of the Brownian motion. In the case of sublimation from the frozen dispersion the pedesis is supressed completely and the contact velocity of the particles is given by the speed of thickening of the surface layer during the sublimation. B) The specific surface energy and ζ-potential. The ζ-potential of the magnitude larger than approximately 30 mv is assumed to prevent the aggregation of the colloid particles due to Coulomb repulsion. If the magnitude of the ζ-potential is smaller, the aggregation is significant and the dispersed particles can occur in the immediate vicinity of one another. At the end of evaporation the activity of surface atoms can result into even more stable Van der Waals bonds. C) The frequency of the collisions, mutual orientation and the size of interfaces. These factors of the dispersion are of statistical nature. However, the case of very dense dispersion of the filtered cake is much more advantageous for creation of the bonds. The collisions are frequent and close spacing supports the optimal interface orientation. In this case relatively dense aggregates with smaller specific surface area preferentially grow. The basic principle of the preparation of the fibrous and lamellar microaggregates by the vacuum freezedrying method is the minimization of the kinetic energy of the collision (ad A)) in favour of the bonds of the surface atoms (ad B)). The maximum speed of the approaching particles equals the very low speed of the sublimation boundary shift. The speed and the density of the outlet vapour flow, which according to the observed properties can well be called a sublimation wind, also considerably affect the creation of the bonds between the particles. The speed of the phase border retraction and therefore the maximum speed of the two nanoparticles convergence due to sublimation were estimated from the empirical formula for the sublimation pressure IAPWS [6.] 3 b 1 M é i H2O ćt ö vo ( T ) = pt ( 1 - k ) exp ai r ice 2pRT ęĺ ç i = 1 çč T ë tř ě a1 = - 21, 214; a2 = 27, 320; a3 = - 6,106; b1 = 0, 0033; b2 = 1, 207; b3 = 1, 703ü ď ď í ý ďpt = 611, 657 Pa; T t = 273,16 K; r ice = 926 kgm ; M H 0, 018 kgmol 2O = î ďţ - 1 ů úű (1) In Fig. 1 there is shown the relation of the speed vo ( t ) of the sublimation phase border retraction and the temperature t at the vacuum depth equal to 96 % of saturated steam pressure at that temperature.

3 1,E+02 1,E+01 vo [µm s -1 ] 1,E+00 1,E-01 1,E t [ o C] Fig. 1 The temperature dependence vo ( t ) of the speed of the sublimation phase border retraction according to (1). The depicted curve holds for the relative vacuum coefficient of k = We studied the speed of the sublimation in the recipient above the freezing chamber of the lyophilizer with the vacuum aperture of 12.6 cm 2 at the temperature 60 C. Under the given experimental setup and the partial water vapour pressure 12 Pa we measured the sublimation mass decrease m from a free circular surface of 13 mm the diameter (see Fig. 2). Under the described conditions the thermodynamic equilibrium between the sublimation and resublimation flows was achieved at the surface temperature of 40 C.

4 5,0 4,0 m [g] 3,0 2,0 1,0 0, Fig. 2 The sublimation mass loss from the 531 mm 2 frozen area at the surface temperature 40 C as a function of time. τ [s] The line slope leads to the speed of the sublimation interface retraction v exp (T = K) = 0.61 µm s -1. When the water molecules sublimate from the frozen dispersion surface, most of the left-over nanoparticles are fixed to the vacuum-ice interface. They are then drifted together with the retracting interface towards the other nanoparticles in the remaining volume of the dispersion. Consider a model of spherical silicon nanoparticles with the diameter of d = 100 nm, then the maximum contact kinetic energy reaches the value ( Si ) 1 1 =». (2) max E k r pd vo J During the ordinary drying of a wet filtration cake at 60 C the mean kinetic energy of the nanoparticles is given by their Brownian motion in thick, yet still liquid dispersion. According to the experimental analysis of Br the Brownian particles motion in [7.], the mean kinetic energy E k of the particles in liquid can be estimated by a hydrodynamic analogy to the kinetic theory as Br 1 m E k k T 2 m - 22 = B» J. (3) * The quotient m / m *» 0.5 is the ratio of the mass of an isolated Brownian particle to its effective mass taking account of inertial effects of the surrounding liquid. Thus the kinetic energy of the Brownian motion is about nine orders higher than the contact energy at controlled vacuum freeze-drying regime. Assuming a perfect contact of spherical particles on 0.1% of the surface (κ = 0.001), the specific surface energy of silicon A Si = 1.82 J m -2 yields the estimate of the binding energy between two particles 2-17 D E = Ak pd» J. (4) The binding energy is predominant over the kinetic energy for both the methods of drying. There is, however, considerable difference in frequencies of mutual collisions of the particles. High collision frequency of

arrangement where the possible self-assembly effects at micro scale are suppressed.

nanoparticles supports the creation of the stable bounds at the first contact without further disturbing

The undisturbed self-assembly process applies much more effectively here and larger fibrous and lamellar

EXPERIMENTAL RESULTS An aqueous dispersion of the disintegrated silicon nanoparticles with the diameter median of

5 , Brno, Czech Republic Brownian particles in a very thick filtration cake will lead to mostly chaotic tight arrangement where the possible self-assembly effects at micro scale are suppressed. On the other hand, during the controlled vacuum freeze-drying process, the sparse distribution of the nanoparticles supports the creation of the stable bounds at the first contact without further disturbing interactions. The undisturbed self-assembly process applies much more effectively here and larger fibrous and lamellar aggregates are created. 2. EXPERIMENTAL RESULTS An aqueous dispersion of the disintegrated silicon nanoparticles with the diameter median of 148 nm was vacuum freeze-dried in the controlled regime described above. Fig. 3 The fibrous microstructure aggregated from silicon nanoparticles (a source material for semiconductor production) with the diameter median of 148 nm. The picture on the top shows the macroscopic appearance of the fibrous material in a metallic sublimation vessel. The bottom picture is the Field Emission (SEM) micrograph image of the mainly fibrous structure of the silicon aggregates.

6 While the specific surface of the standardly prepared nanopowder was 4 m 2 g -1 only, our product depicted in Fig. 3, which was obtained from the same matter by the controlled vacuum freeze-drying process, shows the markedly higher value of m 2 g DISCUSSION AND CONCLUSIONS The paper describes the preparation method of the fibrous and lamellar microaggregates with high specific surface area by means of controlled vacuum freeze-drying. On the basis of the semi-empirical model of sublimation equilibrium of water at 40 C and 12 Pa and the experimental data, we determined the speed of sublimation interface retraction as 0.61 µm s -1. This is also the speed at which the nanoparticles fixed on the interface approach the rest of the nanoparticles still dispersed inside the volume of the ice. Their kinetic energy is nine orders less than the kinetic energy of the Brownian motion of the nanoparticles in the thick aqueous dispersion in a filtration cake at 60 C. In both the cases the kinetic energies are much less than the estimate of the binding energy at minimal surface contact of the particles. However, during the vacuum freeze-drying the collision frequency is much lower which enables undisturbed self-assembly process and larger aggregates to grow. Using the above described method, we transformed semiconductive silicon nanoparticles into fibrous and lamellar structures with high specific surface area of m 2 g -1. Altering the morphology of particles, the temperature, and the partial pressure of water vapor in vacuum influences the size and the structure of the final product. Further optimization of these parameters will probably result into even higher values of the specific surface area. AKNOWLEDGEMENTS This research was performed at VŠB-Technical University of Ostrava, sponsored by the Czech National Grant Agency (GAČR) under the project P107/11/1918 and the Regional Material Technology Research Centre (RMTVC) under the project CZ.1.05/2.1.00/ REFERENCES [1.] Chikara, H., Seiichiro, K., Masaaki, O., Fumio, N.: The use of nanoparticles as coatings, Materials Science and Engineering: A, Volume 163, Issue 2, Containing papers presented at the 12th International Vacuum Congress, 15 April 1993, Pages , ISSN , DOI: / (93) [2.] Granqvist, C.G., Buhrman, R. A., Wyns, J., Sievers, A. J.: Far-Infrared Absorption in Ultrafine Al Particles, Phys.Rev.Lett. 37 (1976), p [3.] Praus, P., Kozák, O., Kočí, K., Panáček, A., Dvorský, R.: CdS nanoparticles deposited on montmorilonite: Preparation, characterization and application for photoreduction of carbon dioxide, Journal of Colloid and Interface Science 360 (2011) [4.] Praus, P., Turicova, M., Studentova, S., Ritz, M.: Study of cetyltrimethylammonium and cetylpyridinium adsorption on montmorillonite, Journal of Colloid and Interface Science, Volume 304, Issue 1, 2006, Pages [5.] Dvorský, R., Luňáček, J., Slíva, A.: Dynamics Analysis of Microparticles Cavitation Disintegration During Nanopowder Preparation in New Water Jet Mill (WJM), Advanced Powder Technology 2010 in press - doi: /j.apt [6.] IAPWS document 2008, Revised Release on the Pressure along the Melting and Sublimation Curves of Ordinary Water Substance, [7.] Li, T., Kheifets, S., Medellin D., et al.: Measurement of the Instantaneous Velocity of a Brownian Particle, SCIENCE Volume 328, Issue 5986, Pages

CAVITATION DEPOSITION OF ZnS NANOPARTICLES INTO COMPOSITE WITH MONTMORILLONITE PARTICLES

CAVITATION DEPOSITION OF ZnS NANOPARTICLES INTO COMPOSITE WITH MONTMORILLONITE PARTICLES Richard DVORSKÝ, Petr PRAUS, Jana TROJKOVA, Soňa ŠTUDENTOVÁ, Jiří LUŇÁČEK VSB-Technical University of Ostrava, Ostrava,